Switching devices have been widely used, such as an intelligent switch and a relay switch, for selectively feeding the power supply to the loads. In recent years, semiconductor switches are mainly using diodes, bipolar transistors and MOSFETs (metal oxide semiconductor field-effect transistors) on account of their durability to substitute for the mechanical switches.
There has been proposed a semiconductor switch including a light-emitting element that emits light in response to input signals, a light-receiving element which produces an electromotive force upon receiving light, a charge/discharge circuit for effecting the charge/discharge control operation depending upon the electromotive force, and an output semiconductor element which varies the impedance depending upon the charge/discharge control operation, and further having a temperature sensor which detects the temperature of the output semiconductor element and produces an interrupting signal, and an interrupting circuit for interrupting the input signal upon receipt of the interrupting signal (see JP-A-2003-218382).
There has further been proposed an intelligent power module including a semiconductor switch and various protection circuits mounted on the same substrate, which detects protection signals produced from various protection circuits, and produces processing signals of forms that differ depending upon the kind of the detected protection signal, which in turn differs according to the degree of trouble that has occurred, so that a suitable processing (on/off control of the semiconductor switch, etc.) can be executed at the receiving side depending upon the degree of trouble (see JP-A-2003-88093).
Further, there has been proposed an intelligent power switch and a switching device having an overcurrent protection function and an overheat protection function in order to quickly judge whether the semiconductor switch is turned off or whether the load is opened so that parts can be quickly replaced or repaired. The device is capable of quickly judging if no power supply is fed to the load due to the protection function causing the semiconductor switch to be turned off or if no power supply is fed to the load due to because the load is opened, relying upon a simple constitution (see Japanese Patent No. 3285120).
The semiconductor switch switches between outputting and not outputting voltage from the power supply to, for example, an actuator, based on a control signal such as output voltage from a microcomputer or the like. However, because the semiconductor switch cannot judge whether the microcomputer is properly operating, a separate monitoring means to monitor output voltage from the microcomputer is separately installed. This monitoring means stops output to the actuator if voltage supplied to the microcomputer exceeds a certain value. The semiconductor switch also stops output if there is excessive voltage from the power supply, through a self protection function.
According to a method of separately providing monitoring means (or circuit) for monitoring a voltage applied to a microcomputer, the number of parts increases by the provision of the monitoring circuit, thereby causing an increase in the cost of production. Therefore, a circuit board mounting a microcomputer, a semiconductor switch and any other circuitry often employs a method of mutual monitoring, such as exchanging predetermined signals between the microcomputer and a power supply circuit, an input signal processing circuit, an actuator drive circuit or an IC which includes them is required.
However, it is a trend to form ICs of these power supply circuit, input signal processing circuit and actuator drive circuit integrally with their peripheral circuits, making it difficult to realize the mutual monitoring. At present, there have been proposed many ICs that are isolated and arranged depending upon their functions enabling the mutual monitoring to be accomplished. However, with the mutual monitoring in the same IC, it is difficult to guarantee perfect independence between the monitoring side and the to-be-monitored side, which may not be considered to be reliable enough in the fields where the safety is very important (such as controlling the brakes of automobiles).
It is not possible for the conventional semiconductor switches to judge whether the input signals are normal when the mutual monitoring cannot be performed sufficiently. Therefore, even when the microcomputer operates abnormally, the switching is executed according to signals output from the microcomputer. Therefore, depending upon the switching state, the circuit or the actuator connected to the semiconductor switch is often damaged.